DC to DC converters are frequently employed to convert relatively low voltage DC sources into high voltage DC sources. The high voltage DC sources are then suitable for application to a DC load, such as electrodes of an electron tube or other electrical devices.
Referring to FIG. 2, a typical DC to DC converter 1 includes a first rectifying and filtering circuit 11, a protecting circuit 13, a transformer 15, a second rectifying and filtering circuit 16, a pulse width modulation (PWM) circuit 17, a rectifying diode 18, and a transistor 19. The transistor 19 is a p-channel metal-oxide-semiconductor field-effect transistor (P-MOSFET).
The PWM circuit 17 includes a voltage input 171 configured to receive an operation voltage and a pulse output 172 configured to provide a pulse signal to a gate electrode of the transistor 19.
The first rectifying and filtering circuit 11 includes two inputs 111, 112 configured to receive an external alternating current (AC) voltage such as a 220V AC voltage, a full-bridge rectifying circuit 110 configured to convert the 220V AC voltage to a first direct current (DC) voltage, a filter capacitor 114 configured to stabilize the first DC voltage, and a first output 113 configured to provide the first DC voltage to the transformer 15. Two inputs of the full-bridge rectifying circuit 110 serve as the two inputs 111, 112. A positive output of the full-bridge rectifying circuit 110 serves as the first output 113. A negative output of the full-bridge rectifying circuit 110 is connected to ground. The filter capacitor 114 is connected between the first output 113 and ground.
The transformer 15 includes a primary winding 151, an assistant winding 152, and a secondary winding 153. The first output 113 of the first rectifying and filtering circuit 11 is connected to ground via the primary winding 151, a drain electrode and a source electrode of the transistor 19, and a resistor 190 in series. The gate electrode of the transistor 19 is connected to the pulse output 172 of the PWM circuit 17. The protecting circuit 13 is connected in parallel with the primary winding 151.
One terminal of the assistant winding 152 is connected to ground. The other terminal of the assistant winding 152 is connected to the voltage input 171 of the PWM circuit 17 via the anode and the cathode of the rectifying diode 18 in serials.
The secondary winding 153 is coupled to a second output 163 via the second rectifying and filtering circuit 16 for providing a second DC voltage to a load circuit (not shown) through the second output 163.
The external AC voltage is provided to the two inputs 111, 112 of the first rectifying and filtering circuit 11 and is transformed into the first DC voltage by the first rectifying and filtering circuit 11. Then the first DC voltage is provided to the primary winding 151. The assistant winding 152 induces the primary winding 151, generates an operation voltage, and provides the operation voltage to the voltage input 171 of the PWM circuit 17 via the rectifying diode 18. Thus the PWM circuit 17 generates the pulse signal for switching on or switching off the transistor 19. When the transistor 19 is switched on, a first current path is formed sequentially through the first output 113, the primary winding 151 of the transformer 150, the transistor 19, and the resistor 190. A first current is formed when the first DC voltage provided to the first output 113 is connected to ground via the first current path. The first current flowing through the first current path linearly increases until electromagnetic induction generated in the primary winding 151 reaches a predetermined maximum threshold.
When the transistor 19 is switched off, the energy stored in the primary winding 151 of the transformer 150 transfers to the secondary winding 153. Thus the second DC voltage is generated and provided to the load circuit. A rush current flowing through the primary winding 151 can be depressed by the protecting circuit 13.
However, because the PWM circuit 17 is an expensive component, the cost of the DC to DC converter 100 is correspondingly high.
It is desired to provide a new DC to DC converter which can overcome the above-described deficiencies.